We got evidence of magicla girls living to their late 20s (mayeb she just stopped fighting witches at some point, we don't have that data), though the one we have evidence for eventually becomes a witch too, possibly becasue of the stress of adult life.

Suppose a really proficient Witch-killer/Grief Seed-collector managed to collect hundreds upon hundreds of Grief Seeds (I recall a game screenshot of Kyouko where she's hoarding Grief Seeds like Scrooge McDuck hoards gold coins ). With enough Grief Seeds, you could have a "lifetime supply" that could see you last 50, 60, 70 years (presuming you keep magic use down to a minimum).

Eventually (if possible), you die a natural death. Or, if the aging process is drastically different for Puella Magi, maybe you could go hunting more witches again and/or accept that you've lived a normal lifespan and commit suicide with a bullet through your Soul Gem.

This is probably the closest to "Winning!" (in a Charlie Sheen sense ) that a Puella Magi could hope to achieve.